An exiled Iranian opposition group today claimed to have evidence of a hidden nuclear site located in tunnels beneath a mountain near the town of Damavand, 70 kilometres northeast of Tehran.

An exiled Iranian opposition group on Thursday claimed to have evidence of a hidden nuclear site located in tunnels beneath a mountain near the town of Damavand, 70 kilometres northeast of Tehran.

The Paris-based militant group, the People's Mujahedin of Iran (MEK), alleges the site has existed since 2006 with the first series of sub-terranean tunnels and four external depots recently completed.

The group also claims the recently elected President Hassan Rohani, a former nuclear negotiator, had a "key role" in the programme.

Founded in the 1960s to oppose the rule of the Shah, the MEK was considered a terrorist organisation by the United States until 2012, and has provided information about the Iranian nuclear programme on several occasions.

"The organisation of the People's Mujahedin of Iran (MEK) has discovered credible evidence of a secret new nuclear site, gathered over a year by 50 sources in various parts of the regime," said a statement from the National Council of Resistance of Iran, the umbrella group of which MEK is a part.

"The codename of the project is 'Ma'adane-e Charq' (literally 'the mine of the east') or 'Project Kossar'. This site is hidden in a series of tunnels under a mountain near the town of Damavand," it said.

The report added that Mohsen Fakhrizadeh, a senior official in Iran's Revolutionary Guard, is also a managing director of a company the MEK claims is overseeing the project's "nuclear, biological and chemical programmes."

The International Atomic Energy Agency (IAEA) has attempted to speak to Fakhrizadeh in the past without success. The "next phase" of the project will be the construction of up to 30 tunnels and 30 depots, the report added.

The report concluded: "These revelations demonstrate once again that the Mullahs' regime has no intention of stopping or even suspending the development of a nuclear weapon," the MEK said, calling on the IAEA to visit the secret site.